There has conventionally been a high demand for optical information recording mediums to have high density and large capacity, due to the development in information communication technology and multimedia technology as well as the highly information-based trend. However, an upper limit of the recording density in an optical information recording medium is limited generally by a spot diameter of an optical beam that records or reproduces information. This is because a reduction in length of a recording mark diameter so as to attain high density in the optical information recording medium causes a spot region to include a plurality of marks, thereby causing difficulty in separately detecting the marks. The spot diameter of the optical beam is substantially represented by λ/NA, where a wavelength of a light source is λ, and a numerical aperture of the objective lens for forming an optical spot is NA. Therefore, it has been believed that a recording density of an optical information recording medium is improvable by reducing the spot diameter of the optical beam, which is reduced by shortening a length of a wavelength λ of the light source and increasing the numerical aperture NA of the objective lens.
However, since (i) a wavelength of an ultraviolet region is thought to be a limit of a wavelength λ of a light source in view of absorption of optical elements and sensitivity properties of a detector of a reproducing apparatus for reproducing the optical information recording medium, and (ii) the increase in numerical aperture NA of an objective lens in a reproducing apparatus for an optical information recording medium is substantially limited due to an allowed amount of tilt to the optical information recording medium, improvement in the recording density of an optical information recording medium by reducing a spot diameter of an optical beam is limited.
In recent years, development of an optical information recording medium that uses a super-resolution reproduction technique has been advancing. The super-resolution reproduction technique is a technique capable of reproducing a mark length not longer than a resolution limit (optical resolving power determined by a diffraction limit of reproducing light) of a reproducing optical system (optical system of a reproducing apparatus for reproducing an optical information recording medium). Use of such a super-resolution reproduction technique makes it possible to reproduce a mark length not longer than a resolution limit of the reproducing optical system. Thus, recording to the optical information recording medium by using a shorter mark length is possible. That is to say, it is substantially possible to increase a recording density of the optical information recording medium by use of the foregoing super-resolution reproduction technique. Hereinafter, an optical information recording medium which utilizes the foregoing technique is called a super-resolution reproducing medium, and reproduction which reproduces, by use of the foregoing technique, a recording pit of a mark length not longer than the resolution limit of an reproducing optical system is called super-resolution reproduction.
A resolution limit of a reproducing optical system, which is restricted by a frequency limit of a detectable signal, is generally said to be around λ/(2NA) (λ: reproducing light wavelength; NA: aperture ratio of lens). However, this λ/(2NA) is equivalent to a resolution limit of a period size of a pattern made by a repetition of a single-sized recording mark and a single-sized space, and λ/(4NA), which is half of the λ/(2NA), is known as a resolution limit of a recording mark length. Thus, hereinafter, a resolution limit denotes a resolution limit of a length of a recording mark, and such a resolution limit is λ/(4NA). Practically, the resolution limit is effected by other elements in the optical system other than theory, therefore the resolution limit value may differ from a theoretical value calculated from a wavelength and a numerical aperture.
As an example of a technique which is capable of carrying out the aforementioned “super-resolution reproduction” which exceeds the resolution limit, Patent Literature 1 discloses an optical information recording medium capable of carrying out super-resolution reproduction by providing a functional layer on an uneven information recording surface, which functional layer improves spatial resolving power and is made of a simplex, an alloy, or a compound of metal, semiconductor material or like material (however, a specific reproduction principle of such an optical information recording medium is unknown). The optical information recording medium disclosed in Patent Literature 1 employs a method which disposes an identically-shaped mark not longer than the resolution limit in a direction of signal reproduction. More specifically, a single frequency repetitive phase pit (mark space ratio 1:1) is reproduced, so as to evaluate the system based on a CNR (carrier-to-noise ratio). The evaluation resulted in that a recording density was improved, thereby making super-resolution reproduction possible. The aforementioned pit pattern is hereinafter called a monotone pattern.
On the other hand, a mark edge recording system is generally employed as means for attaining high density of an optical information recording medium in view of signal processing. A mark edge recording system is a system in which high density is attained in an optical information recording medium by using both ends of one recording mark as signals. In this system, a shortest pre-pit which has a shortest mark length in a reproduction beam scanning direction and pre-pits in several lengths based on the shortest length are specified by standards, and the pre-pits of different lengths are disposed in order in a direction of signal reproduction, following a rule specified by the standards. The aforementioned pit patterns are hereinafter called random patterns.
There are many practically used examples of the mark edge recording system that use a random pattern. For example, in case of a CD (Compact Disc), an EFM (8/14) (Eight to Fourteen Modulation) is employed as a modulation system. For other mediums such as a DVD (Digital Versatile Disk), BD (Blu-ray Disc), and HD DVD (High-Definition Digital Versatile Disk), the modulation system for each of these mediums are different from that of the CD: the DVD employs EFM Plus (8/16); the BD employs RLL (1,7); and the HD DVD employs ETM (8/12) (Eight to Twelve Modulation).
That is to say, with many of the optical information recording mediums, a mark edge recording system using a random pattern is employed. This is because, recording with use of the mark edge recording system of random patterns allows recording of information with higher density as compared to recording with use of a monotone pattern. Consequently, in order to apply the recording medium into practical use, there is the need to evaluate the optical information recording medium that uses the random pattern. An index of the evaluation may be, for example, a bER (Bit Error Rate). The bER is also called a bit error rate, and is a ratio of an error bit number Ne with respect to a whole decoded bit number Nt. The error bit number Ne is included in a decoded result of a signal which is attained as a result of reproducing a random pattern recorded in the optical information recording medium. The ratio is represented by an equation of bER=Ne/Nt.
Reproduction of an optical information recording medium recorded in high density by the mark edge recording system that uses random pattern uses a PRML (Partial Response Maximum Likelihood) decoding. Conventional optical information recording mediums generally uses peak detect as a signal detection method, however the PRML decoding is practically essential for the recent high density optical information recording mediums such as BD and HD DVD, and therefore is generally used.
An example of the PRML is PR(12221)ML used for the HD DVD. Moreover, for example, Non Patent Literature 1 discloses a signal decoding method of a super-resolution reproducing method called Super-BENS, for improving a bER property of a random pattern super-resolution reproduction. More specifically, Non Patent Literature 1 discloses a signal processing which is an advancement to the PRML signal decoding method for improving a bER property of super-resolution reproduction of random pattern.